Nomadic Planets Near the Solar System: Detecting the Natural Early Targets for Interstellar Exploration (original) (raw)

Comments Astrophys., 1990

The presence of unseen mass in the solar neighbourhood has prompted modelling of, and searches for, a population of cool, low mass stars to make up the deficit. Such brown dwarfs are thought to exist within a mass range of 0.01 M⊙ < M < 0.08 M⊙. In this paper the possibility of the existence of interstellar planets (ISPs) of mass range 5x10^-9 M⊙ < M < 0.01 M⊙ is examined. Six potential modes of formation of ISPs are identified, although some are mutually exclusive, depending of different cosmogonic hypotheses. ISPs are of two basic types: those formed solitary within molecular clouds and those formed within, and subsequently unbound from, planetary systems. While the existence of the former is uncertain, interstellar planets of the unbound variety almost definitely exist, although not in sufficient quantity to account for the unseen mass. The number density of unbound planets in the solar neighbourhood may be of a similar, or greater, order of magnitude to that of stars, the majority of them being massive planetesimals ejected from planetary systems in formation. The nearest extra-solar planet may thus be closer to the solar system than the nearest star.